Pension and Other Postretirement Benefits (Schedule of Components of Other Postretirement Benefit Expense) (Details) - Medical and Life Insurance Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Postretirement medical and life insurance benefit expense
Service cost	$ 1	$ 1	$ 1
Interest cost	1	2	1
Amortization of prior service cost (credit)	(3)	(3)	(3)
Amortization of net actuarial loss (gain)	1	1	1
Net cost	$ 0	$ 1	$ 0